Lanthanum (La) is a chemical element with atomic number 57. Its only stable isotope is ¹³⁹La, with 57 protons and 82 neutrons.

Lanthanum is the first element in the element series called lanthanides which stretches from lanthanum to lutetium. These fifteen elements, together with scandium and yttrium are collectively called rare earth metals, and all have very similar properties.